// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
using System;
using System.Collections.Immutable;
using System.Threading;
using Microsoft.CodeAnalysis.CSharp.Extensions;
using Microsoft.CodeAnalysis.CSharp.LanguageService;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.Shared.Collections;
using Microsoft.CodeAnalysis.Shared.Extensions;
using Microsoft.CodeAnalysis.Text;
 
namespace Microsoft.CodeAnalysis.CSharp.Analyzers.RemoveUnnecessaryNullableDirective
{
    internal sealed class NullableImpactingSpanWalker : CSharpSyntaxWalker, IDisposable
    {
        private readonly SemanticModel _semanticModel;
        private readonly int _positionOfFirstReducingNullableDirective;
        private readonly SimpleIntervalTree<TextSpan, TextSpanIntervalIntrospector>? _ignoredSpans;
        private readonly CancellationToken _cancellationToken;
 
        private ImmutableArray<TextSpan>.Builder? _spans;
 
        public bool HasSpans => _spans?.Count > 0;
 
        public ImmutableArray<TextSpan> Spans => _spans?.ToImmutable() ?? ImmutableArray<TextSpan>.Empty;
 
        public ImmutableArray<TextSpan>.Builder SpansBuilder
        {
            get
            {
                if (_spans is null)
                    Interlocked.CompareExchange(ref _spans, ImmutableArray.CreateBuilder<TextSpan>(), null);
 
                return _spans;
            }
        }
 
        public NullableImpactingSpanWalker(
            SemanticModel semanticModel,
            int positionOfFirstReducingNullableDirective,
            SimpleIntervalTree<TextSpan, TextSpanIntervalIntrospector>? ignoredSpans,
            CancellationToken cancellationToken)
            : base(SyntaxWalkerDepth.StructuredTrivia)
        {
            _semanticModel = semanticModel;
            _positionOfFirstReducingNullableDirective = positionOfFirstReducingNullableDirective;
            _ignoredSpans = ignoredSpans;
            _cancellationToken = cancellationToken;
        }
 
        private bool IsIgnored(SyntaxNode node)
        {
            if (node.Span.End < _positionOfFirstReducingNullableDirective)
                return true;
 
            if (_ignoredSpans is not null)
            {
                if (_ignoredSpans.HasIntervalThatContains(node.SpanStart, node.Span.Length))
                {
                    return true;
                }
            }
 
            return false;
        }
 
        private static bool IsLanguageRestrictedToNonNullForm(TypeSyntax node)
        {
            // Simplify syntax checks by walking up qualified names to an equivalent parent node.
            node = WalkUpCurrentQualifiedName(node);
 
            if (node?.Parent is QualifiedNameSyntax qualifiedName
                && qualifiedName.Left == node)
            {
                // Cannot dot off a nullable reference type
                return true;
            }
 
            if (node.IsParentKind(SyntaxKind.UsingDirective))
            {
                // Using directives cannot directly reference a nullable reference type
                return true;
            }
 
            if (node.IsParentKind(SyntaxKind.SimpleBaseType))
            {
                // Cannot derive directly from a nullable reference type
                return true;
            }
 
            if (node?.Parent is BaseNamespaceDeclarationSyntax)
            {
                // Namespace names cannot be nullable reference types
                return true;
            }
 
            if (node.IsParentKind(SyntaxKind.NameEquals) && node.Parent.IsParentKind(SyntaxKind.UsingDirective))
            {
                // This is the alias or the target type of a using alias directive, neither of which can be nullable
                //
                //   using CustomException = System.Exception;
                //         ^^^^^^^^^^^^^^^   ^^^^^^^^^^^^^^^^
                return true;
            }
 
            return false;
 
            // If this is Y in X.Y, walk up to X.Y
            static TypeSyntax WalkUpCurrentQualifiedName(TypeSyntax node)
            {
                while (node.Parent is QualifiedNameSyntax qualifiedName
                    && qualifiedName.Right == node)
                {
                    node = qualifiedName;
                }
 
                return node;
            }
        }
 
        public void Dispose()
        {
        }
 
        public override void DefaultVisit(SyntaxNode node)
        {
            if (IsIgnored(node))
                return;
 
            if (node is TypeSyntax typeSyntax
                && !IsLanguageRestrictedToNonNullForm(typeSyntax))
            {
                if (typeSyntax.IsVar)
                    return;
 
                if (typeSyntax is PredefinedTypeSyntax predefinedType
                    && CSharpSyntaxFacts.Instance.TryGetPredefinedType(predefinedType.Keyword, out var type))
                {
                    if (type is CodeAnalysis.LanguageService.PredefinedType.Object or CodeAnalysis.LanguageService.PredefinedType.String)
                    {
                        SpansBuilder.Add(predefinedType.Span);
                    }
 
                    // All other predefined types are value types
                    return;
                }
 
                var symbolInfo = _semanticModel.GetSymbolInfo(typeSyntax, _cancellationToken);
                if (symbolInfo.Symbol.IsKind(SymbolKind.Namespace))
                {
                    // Namespaces cannot be nullable
                    return;
                }
                else if (symbolInfo.Symbol is INamedTypeSymbol { IsValueType: true, IsGenericType: false })
                {
                    return;
                }
 
                SpansBuilder.Add(node.Span);
                return;
            }
 
            base.DefaultVisit(node);
        }
    }
}